Spontaneous insertion of plant plasma membrane (H⁺)ATPase into a preformed bilayer

Summary

The purified (H⁺ATPase from corn roots plasma membrane inserted spontaneously into preformed bilayer from soybean lipids. The yield of the protein insertion, as measured from its H⁺-pumping activity, increased as a function of lipids and protein concentrations. In optimum conditions, all the (H⁺)ATPase molecules were closely associated with liposomes, exhibiting a high H⁺-pumping activity (150,000% quenching· min⁻¹·mg⁻¹ protein of the probe 9-amino-6-chloro-2-methoxyacridine). The insertion was achieved within a few seconds. No latency of the (H⁺)ATPase hydrolytic activity was revealed when lysophosphatidylcholine was added to permeabilize the vesicles. This indicated that the (H⁺)ATPase molecules inserted unidirectionally, the catalytic sites being exposed outside the vesicles (“inside-out” orientation), and thus freely accessible to Mg-ATP. The nondelipidated (H⁺)ATPase could also functionally insert into bilayer from PC∶PE∶PG or PC∶PE∶PI, due to the presence of both hydrophobic defects promoted by PE, and negative phospholipids specifically required by the (H⁺)ATPase from corn roots. The detergent octylglucoside facilitated the delipidated (H⁺)ATPase reinsertion probably by promoting both a proper protein conformation and hydrophobic defects in the bilayer. Lysophosphatidylcholine facilitated the delipidated protein insertion only when hydrophobic defects were already present, and thus seemed only capable to ensure a proper protein conformation